Practical and efficient synthesis of orthogonally protected α-2,3-diaminopropionic acid (2,3-Dap), 2,4-diaminobutanoic acid (2,4-Dab), and their N-methylated derivatives

Article abstract of DOI:10.1080/00397910600772827

The synthesis of orthogonally protected Fmoc-Dap/Dab (Boc/Z/Alloc)-OH starting from Fmoc-Asp/Glu has been described. The salient features of our synthetic strategy involved formation of Fmoc-Asp/Glu-5-oxazolidinone acids, conversion of acid function to acyl azides, Curtius rearrangement, and hydrolysis of the oxazolidinone group. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600772827

This article was downloaded by: [University of Western Ontario]
On: 12 November 2012, At: 00:41
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office:
Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An International
Journal for Rapid Communication of
Synthetic Organic Chemistry
Publication details, including instructions for authors and subscription
information:
http://www.tandfonline.com/loi/lsyc20
Practical and Efficient Synthesis
of Orthogonally Protected
α‐2,3‐Diaminopropionic Acid (2,3‐Dap),
2,4‐Diaminobutanoic Acid (2,4‐Dab), and
their N‐Methylated Derivatives
R. V. Ramana Rao a , Subramanyam J. Tantry a & Vommina V. Suresh Babu a
a Department of Studies in Chemistry, Central College Campus, Bangalore
University, Bangalore, India
Version of record first published: 16 Feb 2007.
To cite this article: R. V. Ramana Rao, Subramanyam J. Tantry & Vommina V. Suresh Babu (2006):
Practical and Efficient Synthesis of Orthogonally Protected α‐2,3‐Diaminopropionic Acid (2,3‐Dap),
2,4‐Diaminobutanoic Acid (2,4‐Dab), and their N‐Methylated Derivatives, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:19, 2901-2912
To link to this article: http://dx.doi.org/10.1080/00397910600772827
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial
or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or
distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the
contents will be complete or accurate or up to date. The accuracy of any instructions, formulae,
and drug doses should be independently verified with primary sources. The publisher shall not
be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or
howsoever caused arising directly or indirectly in connection with or arising out of the use of this
material.

Synthetic Communicationsw, 36: 2901–2912, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600772827
Practical and Efficient Synthesis of
Orthogonally Protected
a-2,3-Diaminopropionic Acid (2,3-Dap),
2,4-Diaminobutanoic Acid (2,4-Dab), and
their N-Methylated Derivatives
R. V. Ramana Rao, Subramanyam J. Tantry,
and Vommina V. Suresh Babu
Department of Studies in Chemistry, Central College Campus,
Bangalore University, Bangalore, India
Abstract: The synthesis of orthogonally protected Fmoc-Dap/Dab (Boc/Z/Alloc)-
OH starting from Fmoc-Asp/Glu has been described. The salient features of our
synthetic strategy involved formation of Fmoc-Asp/Glu-5-oxazolidinone acids,
conversion of acid function to acyl azides, Curtius rearrangement, and hydrolysis of
the oxazolidinone group.
Keywords: 2,3-Dap and 2,4-Dab, Fmoc-strategy, microwave irradiation, one-pot
synthesis, N-Me derivatives
INTRODUCTION
2,3-Diaminopropionic acid (2,3-Dap) and 2,4-diaminobutanoic acid (2,4-
Dab) are important lower homologues of the a-amino acid lysine. They
are constituents of several peptide antibiotics such as edeines,[1] tuber-
actinomycins,[2] and bleomycins.[3,4] Dap has been used in the synthesis of
2,8-diaminopurines.[5] In 2003, Kretsinger and Schneider[6] demonstrated
the utilization of mono, di, and trimethyl Dap’s in the folding preferences
Received in the USA February 24, 2006
Address correspondence to Vommina V. Suresh Babu, Department of Studies in
Chemistry, Central College Campus, Bangalore University, Bangalore 560 001,
India. E-mail: hariccb@rediffmail.com
2901

2902
R. V. R. Rao, S. J. Tantry, and V. V. S. Babu
Figure 1. Orthogonally protected lower homologous of lysine.
of proteins. The utilization of Dap and Dab in solid-phase synthesis requires
orthogonally protected Dap and Dab (Fig. 1). Shiori et al.[7] synthesized
Na-BOC-Nb-Z-L-Dap-OMe and Boc-Dap(Z) by modified Curtius reaction
using diphenyl phosphorazidate (DPPA). On the similar lines, a detailed
route for the synthesis of Boc-Dap(Z) starting from Boc-Asp was reported
in 2004.[8] Their attempts for the direct synthesis of Dap with orthogonal
protecting groups Boc and Z groups by trapping the Boc-Asp(NCO)-OBzl
with benzyl alcohol was not successful; however, it led to a cyclized
product (Fig. 2) in 90% yield. In our studies on the angiotensin-converting
enzyme (ACE) inhibitor, we need orthogonally protected Fmoc-Dab/Dab
for the synthesis of various lisinopril analogues by the solid-phase method.
RESULTS AND DISCUSSION
The strategy described herein to synthesize orthogonally protected Dap and Dab
starting from Fmoc-Asp/Glu is given in the Scheme 1. In first step, the side chain
carboxylofthe5-oxazolidinones1a,bwereconvertedtoitsacidazide(2a,b)viaa
mixed anhydride. The acid azides (Caution: prone to explosion) were converted
to the corresponding carbamates 3–5 by treatment with appropriate alcohol.
Among various conditions employed, microwave irradiation–assisted
Curtius rearrangement[9,10] (Caution: prone to explosion) and concomitant
formation of the carbamate has resulted in the 5-oxazolidinone carbamates
3–5 in 88, 90, and 92% yields (Table 1).
Figure 2. Cyclization of Asp with single Na-Boc-protecting group during Curtius
rearrangement.

Synthesis of 2,3-Dap and 2,4-Dab and their Derivatives
2903
Scheme 1. Reagents and conditions: (i) paraformaldehyde, p-TsOH (cat.), tolune; (ii)
NMM, ethyl chloroformate, aq. NaN3; (iii) microwave irradiation for 15 min; (iv) (a)
LiOH, (b) silica gel, reflux.
The 5-oxazolidinone acid azide (2a,b) was subjected to the Curtius
rearrangement under microwave irradiation in toluene to give the crystalline
1
isocyanates (9a,b) (Fig. 3). They were fully characterized by H, 13C, and
mass spectroscopies.
The hydrolysis of the 5-oxazolidinone carbamates 3–5 in the presence
of tbutanol, benzyl alcohol or allylic alcohol at 1108C orthogonally
protected Na-Fmoc-Dap(Boc/Z/Alloc)/Na-Fmoc-Dab(Boc/Z/Alloc).
The 5-oxazolidinone ring present in 3 was cleaved by using 1 N LiOH
(1 eq) in THF, whereas for 4 and 5 the ring was cleaved by refluxing the
reaction mixture on silica gel[12] overnight. The Na-Fmoc-Dap(Z/Alloc/
Boc) were obtained in 83, 86, and 70% yields, respectively. The reductive
cleavage of the 5-oxazolidinone carbamate 4a,b using 1N LiOH in
methanol resulted in the methyl ester of the Dap [Fmoc-Dap(Z)-COOMe]
in 73% yield (Scheme 2).
Table 1. Curtius rearrangement of 5-oxarolidinone acid azide 2a
and concomitant formation of the carbamates Fmoc-Dap/Dab(Z)
4a under various conditions
Entry
Method
Time
Temp. (8C)
Yield (%)
1
2
3
4
D
Ultrasonication
CuCl[11]
MW
12 h
12 h
110
70
45
65
55
90
30 min
15 min
25
110

2904
R. V. R. Rao, S. J. Tantry, and V. V. S. Babu
Figure 3. Isocyanates derived from Na-Fmoc-Asp/Glu-5-oxazolidinone.
The reductive cleavage of 5-oxazolidinone carbamates 4a,b with
standard conditions (using triethylsilane and trifluroacetic acid in dry chloro-
form), gave the crystalline N-methyl analogues of Fmoc-Dap/Dab 11a,b
(Scheme 3).
In order to determine the optical purity of the Fmoc-Dab (Alloc)-OH and
Fmoc-Dap(Boc)-OH optical rotations were measured and compared with
reported values.[13]
CONCLUSION
A common route to the synthesis of both Dap and Dab with orthogonal protec-
tion (Na-Fmoc and Nb/g-BOC/Z/AllOC groups) in four steps starting from
Fmoc-Asp/Glu via their oxazolidinones is developed.
EXPERIMENTAL
All chemicals were commercial products of the best grade available. Melting
points were taken on a Buchi model 150 melting-point apparatus in open
capillaries and are uncorrected. IR spectra were recorded on a Nicolet
model impact 400 D FT-IR spectrometer (KBr pellets, 3 cm21 resolution).
1H and 13C NMR spectra were recorded on a Bruker AMX 300-MHz
Scheme 2. Ring opening using LiOH in methanol.

Synthesis of 2,3-Dap and 2,4-Dab and their Derivatives
2905
Scheme 3. Synthesis of N-methyl Dap and N-methyl Dab.
spectrometer. High-resolution mass spectra (HRMS) were recorded on Q-TOP
micromass mass spectrometer. Optical rotations were measured on JASCO-
DIP-370 digital polarimeter (c 1, CHCl3) at room temperature. The
microwave reaction was carried out in a LG MS 194A microwave oven
producing microwave radiation with a frequency of 2450 MHz.
General Procedure for the Synthesis of (S)-3-Fluorenyl-
9-methoxycarbonyl-5-oxazolidinone Acid Azide (2)
To an ice-cold solution of 1a or 1b (1 mmol) in dry THF (5 mL) were added
N-methylmorpholine (NMM) (0.11 mL, 1 mmol) and IBC-Cl (0.135 mL,
1 mmol). The mixture was stirred at 2108C for 10 min. The resulting reaction
mixture was treated with aq. NaN3 (0.098 g, 1.5 mmol in 1 mL) and stirred for
another 30 min. After the completion of the reaction, the organic layer was evap-
orated and the residue was dissolved in CH2Cl2 (20 mL). It was washed with
10 mL of 5% HCl, 5% aq. NaHCO3, and brine and dried over anhydrous
Na2SO4. Evaporation of the solvent in vacuo and recrystallization of the
residue using ethyl acetate–hexane (1:9) yielded the azide as a crvstalline solid.
Data
9H-9-Fluorenylmethyl (4S)-4-(2-azido-2-oxoethyl)-5-oxo-1,3-oxazlone-3-
carboxylate (2a). 0.366 g, 94%, white solid, mp: 628C; [a]2D5 þ 93.5; IR
(KBr): 2155, 1803, 1713, 1645 cm21 1H NMR (300 MHz, CDCl3): 2.3
;
(2H, br, CH2), 4.23 (1H, t, CH), 4.36 (lH, d, CH), 4.41 (2H, d, CH2), 5.1
(2H, s, CH2), 5.5 (1H, br, NH), 7.3–7.75 (8H, m, aromatic); 13C NMR
(75 MHz, CDCl3): 21.2, 47.2, 48.9, 66.8, 119.9, 124.9, 127, 127.6, 141.1,
143.2, 155.8, 156.3, 174.5; HRMS calcd. for C20H16N4O5 ([M þ Na]þ):
þ
415.3594; found: 415.3314.
9H-9Fluorenylmethyl(4s)-4-(3-azido-3-oxopropyl)-5-oxo-1,3-oxazlone-3-car-
boxylate (2b). 0.368 g, 94%, white solid, mp: 808C; [a]2D5 2 72.5, IR (KBr): 2150,

2906
R. V. R. Rao, S. J. Tantry, and V. V. S. Babu
1800, 1703; 1H NMR (300 MHz, CDCl3): 1.5 (2H, m, CH2), 2,3 (2H, m, CH2),
4.23 (1H, t, CH), 4.36 (1H, dd, CH), 4.41 (2H, d, CH2), 5.1 (2H, s, CH2), 5.5 (1H,
br, NH), 7.3–7.75 (8H, m, aromatic); 13C NMR (75 MHz, CDCl3): 21.2, 47.2,
48.9, 66.8, 119.9, 124.9, 127, 127.6, 141.1, 143.2, 155.8, 156.3, 174.5; HRMS
calcd. for C21H18N4Oþ5 ([M þ Na]þ): 429.3862; found: 429.276.
General Procedure for Synthesis of 9
Method A
Compound 2a or 2b (1 mmol) dissolved in toluene (10 mL) was heated at
908C under a nitrogen atmosphere. After 1 h, the solvent was removed
under reduced pressure to get the crude isocyanate 9a and b, which were
recrystallized using ethyl acetate–hexane (1:9).
Method B
Compound 2a or 2b (1 mmol) in toluene (5 mL) in a Teflonw vessel was
exposed to microwave irradiation for 30 s and the remaining solvent was
removed under reduced pressure. Recrystallized using ethyl acetate–
hexane (1:9).
Method C
Powdered compound of 2a or 2b (1 mmol) in a glass beaker was exposed to
microwave irradiation at 60% power until the rearrangement was complete.
The resulting residue was recrystallized.
Method D
Compound 2a or 2b (1 mmol) in toluene (10 mL) was sonicated until the
rearrangement was complete. Then, the solvent was removed under reduced
pressure to obtain the crude isocyanate 9a and b, which were recrystallized.
Data
9H-9-Fluorenylmethyl (4S)-4-(isocyanatomethyl)-5-oxo-1,3-oxazlone-3-
carboxylate (9a). white solid, mp: 898C; [a]2D5 þ 96.5; IR (KBr): 2255,
1
1803, 1713, 1645; H NMR (300 MHz, CDCl3): 3.1 (2H, br, CH2) 4.23
(1H, t, CH), 4.36 (1H, dd, CH), 4.41 (2H, d, CH2), 5.1 (2H, s, CH2), 5.5
(1H, br, NH), 7.3–7.75 (8H, m, aromatic); 13C NMR (75 MHz, CDCl3):
21.2, 47.2, 48.9, 66.8, 119.9, 124.3, 124.9, 127, 127.6, 141.1, 143.2, 155.8,

Synthesis of 2,3-Dap and 2,4-Dab and their Derivatives
2907
156.3, 174.5; HRMS calcd. for C20H16N2Oþ5 ([M þ Na]þ): 387.3454; found:
387.3221.
9H-9-Fluorenylmethyl (4S)-4-(isocyanatomethyl)-5-oxo-1,3-oxazlone-3-car-
boxylate (9b). white solid; mp: 1058C; [a]2D5 2 78.2; IR (KBr): 2251, 1800,
1716, 1111; H NMR (300 MHz, CDCl3): 2.6 (2H, brd, CH2), 1.4 (2H, m,
1
CH2), 4.23 (1H, t, CH), 4.36 (1H, dd, CH), 4.41 (2H, d, CH2), 5.1 (2H, s,
CH2), 5.5 (1H, br, NH), 7.3–7.75 (8H, m, aromatic); 13C NMR (75 MHz,
CDCl3): 21.2, 47.2, 48.9, 66.8, 119.9, 124.3, 124.9, 127, 127.6, 141.1,143.2,
155.8, 156.3, 174.5; HRMS calcd. for C21H18N2Oþ5 ([M þ Na]þ): 401.3722;
found: 401.3455.
General Procedure for the Synthesis of (S)-3-Fluorenyl-
9-methoxycarbonyl-5-oxazolidinone Carbamate (3–5)
The (S)-3-flurenyl-9-methoxycarbonyl-5-oxazolidinone acid azide 2a or 2b
(5 mmol) was dissolved in dry toluene (20 mL), and 3 eq of alcohol (in the
t
case of butanol, 5 eq) was added in a closed Teflonw vessel (250 mL). The
vessel was exposed to microwaves at 40% power for 15 min in four
intervals. After completion of the reaction, the remaining solvent was
removed under reduced pressure and purified by column chromatography
using hexane–ethyl acetate (6:4).
Data
S-3-Fluorenyl-9-methoxycarbonyl-4-[N-tert-butocarbonyl)aminomethyl]-
5-oxazolidinone (3a). 1.96 g, 87%, yellow oil, IR (KBr): 3372, 1803, 1713,
1
1645; H NMR (300 MHz, CDCl3): 1.42 (9H, s, Me3C), 3.45–3.64 (2H, br,
CH2) 4.23 (1H, t, CH), 4.36 (1H, t, CH), 4.36 (1H, dd, CH), 4.41 (2H, d,
CH2), 5.5 (1H, br, NH), 7.3–7.75 (8H, m, aromatic); 13C NMR (75 MHz,
CDCl3): 28.3, 41.2, 47.2, 48.9, 66.8, 119.9, 124.9, 127, 127.6, 141.1, 143.2,
155.8, 156.3, 174.5; HRMS calcd. for C24H26N2Oþ6 ([M þ Na]þ): 461.4674;
found; 461.457.
S-3-Fluorenyl-9-methooxycarbonyl-4-[N-tert-butoxycarbonyl)aminoethyl]-
5-oxazolidinone(3b). 1.985 g, 88%, yellow oil; IR (KBr): 3372, 1803, 1710,
1
1645; H NMR (300 MHz, CDCl3): 1.42 (9H, s, Me3C), 1.55–1.62 (1H, br,
CH), 1.9–2.15 (1H, br, CH), 2.77–2.99 (1H, br, CH), 3.45–3.64 (1H, br,
CH), 4.23 (1H, t, CH), 4.35 (1H, dd, CH), 4.41 (2 H, d, CH2), 5.11 (2H, s,
CH2) 5.4 (1H, br, NH); 13C NMR (75 MHz, CDCl3): 28.3, 47.14,
47.8, 49.8, 52.16, 66.8, 67, 78.9, 119, 124.9, 127, 127.6, 141.3, 143.3, 155,
155.6, 174; HRMS calcd. for C25H28N2Oþ6 ([M þ Naþ): 475.1854; found:
475.4942.

2908
R. V. R. Rao, S. J. Tantry, and V. V. S. Babu
S-3-Flurenyl-9-methoxycarbonyl-4-[N-benzyloxycarbonyl)aminomethyl]-
5-oxazolidinone (4a). 2.17 g, 92%, white solid; mp: 1158C; IR (K Br): 3371,
1800, 1715; 1H NMR (300 MHz, CDCl3): 3.45–3.64 (2H, br, CH2), 4.23 (1H,
t, CH), 4.36 (1H, dd, CH), 4.41 (2H, d, CH2), 5.1 (2H, s, CH2), 5.23–5.32 (2H,
br, CH2), 5.51 (1H, br, NH), 7.3–7.75 (13H, m, aromatic); 13C NMR (75 MHz,
CDCl3): 47.8, 48.7, 56.2, 65.2, 66.5, 78.1, 128.2, 128.5, 135.3, 141.1, 143.24,
155.8, 156.2, 174.5; HRMS calcd. for C27H24N2Oþ6 ([M þ Na]þ): 495.4846;
found: 495.4832.
S-3-Fluorenyl-9-methoxycarbonyl-4-[(N-benzyloxycarbonyl)aminoethyl]-
5-oxazolidinone(4b). 2.21 g, 91%, yellow oil; IR (KBr): 3372, 1803, 1716,
1
1646; H NMR (300 MHz, CDCl3): 1.55–1.62 (1 H, br, CH), 2.15 (1H, br,
CH), 2.7–2.99 (1H, br d, CH), 3.45 (1H, br, CH), 4.23 (1H, t, CH), 4.35
(1H, dd, CH), 4.41 (2H, d, CH2), 5.1 (2H, s, CH2), 5.26–5.31 (2H, br, CH2),
5.4 (1H, br, NH), 7.2–7.8 (13H, m, aromatic); 13C NMR (75 MHz, CDCl3):
47.14, 47.8, 49.18, 52.16, 66.2, 66.8, 67, 119, 124.9, 127, 127.6, 128, 128.5,
135.3, 141.3, 143.8, 155, 155.6, 174; HRMS calcd. for C28H26N2O6þ
([M þ Na]þ): 509.5114; found: 509.1687.
S-3-Fluorenyl-9-methoxycarbonyl-4-[(N-allyloxycarbonyl)aminomethyl]-
5-oxazolidinone (5a). 1.94 g, 92%, white solid, mp: 1308C; IR (KBr): 3371,
1800, 1715; 1H NMR (300 MHz, CDCl3): 3.4–3.64 (2 H, br, CH2), 4.23 (1H,
t, CH), 4.33 (1H, dd, CH), 4.41 (2H, d, CH2), 4.6 (2H, d, CH2), 5.09 (2H, s,
CH2), 5.26–5.3 (1H, m, CH), 5.48 (1H, br, NH), 5.86–5.9 (2H, m, CH2),
7.2–7.8 (8H, m, aromatic); 13C NMR (75 MHz, CDCl3): 40.6, 46.9, 55.2,
65.7, 67.0, 67.5, 117.7, 119.6, 124.5, 126.9, 127.5, 132.4, 141.2, 152.3,
156.2, 170.3; HRMS calcd. for C23H22N2Oþ6 ([M þ Na]þ): 445.4248; found:
445.3842.
S-3-Fluorenyl-9-methoxycarbonyl-4-[(N-allyloxycarbonyl)aminomethyl]-
5-oxazolidinone (5b). 2.02 g, 93%, white solid, mp: 1058C; IR (KBr): 3371,
1804, 1713; 1H NMR (300 MHz, CDCl3): 1.55–1.62 (1H, br, CH), 1.95–2.1
(1H, brd, CH), 3.45–3.64 (1H, brd, CH), 4.23 (1H, t, CH), 4.35 (1H, dd, CH),
4.41 (2H, d, CH2), 4.6 (2H, d, CH2), 5.1 (2H, s, CH2), 5.27 (1H, q, CH), 5.7
(1H, br, NH), 5.88 (2H, m, CH2), 7.2–7.8 (8H, m, aromatic); 13C NMR
(75 MHz, CDCl3): 21.5, 38.2, 44.9, 54.1, 65.7, 67.1, 67.5, 117.8, 119.6,
124.5, 126.9, 127.4, 132.4, 141.2, 152.5, 156.6, 170.4; HRMS calcd. for
C24H24N2Oþ6 ([M þ Na]þ): 459.4514; found: 459.3928.
General Procedure for the Synthesis of 6
To 5 mmol of a 3a or 4a in 10 mL of THF was added 1 eq. of 1 N of LiOH at
room temperature, and the mixture was stirred until the completion of the
reaction. The reaction mixture was concentrated under reduced pressure,

Synthesis of 2,3-Dap and 2,4-Dab and their Derivatives
2909
and the residue was diluted with water (15 mL) and extratex into EtOAc. The
combined organic layer was washed with brine solution and dried
over anhydrous Na2SO4 and purified by column chromatography using
CHCl3–MeOH (99:1).
Data
S-2-FIuorenyl-9-methoxycarbonyl-3-[N-tert-butoxycarbonl]Propionicacid
(6a). 1.49 g, 70%, white solid, mp: 638C; [a]2D5 (c 0.72, MeOH/DCM): 26.1
1
(observed), (reported[13] 2 6.2); IR (KBr): 3372, 1713, 1645, 1111; H NMR
(300 MHz, CDCl3): 1.42 (9H, s, Me3C), 3.45–3.64 (2H, br, CH2) 4.23 (1H, t,
CH), 4.36 (1H, t, CH), 4.36 (1H, dd, CH), 4.41 (2H, d, CH2), 5.5 (1H, br, NH),
6.18 (1H, br, NH), 7.3–7.75 (8H, m, aromatic); 13C NMR (75 MHz, CDCl3):
28.3, 41.2, 47.2, 48.9, 119.9, 124.9, 127, 127.6, 141.1, 143.2, 155.8, 156.3,
174.5; HRMS calcd. for C23H26N2O6þ ([M þ Na]þ): 449.4564; found: 449.448.
S-2-Fluorenyl-9-methoxycarbonyl-4-[N-tert-butoxycarbonyl]butanoic acid
(6b). 1.6 g, 73%, white solid, mp: 1128C; [a]2D5 2 8.6; IR (KBr): 3372, 1713,
1112; H NMR (300 MHz, CDCl3): 1.4 (9H, s, Me3C), 1.9–2.1 (2H, brd,
1
CH2), 2.95–3.52 (2H, brd, CH2) 4.23 (1H, t, CH), 4.35 (1H, dd, CH), 4.41
(2H, d, CH2), 5.2 (1H, br, NH), 5.4 (1H, br, NH), 7.2–7.8 (8H, m, aromatic);
13C NMR (75 MHz, CDCl3): 28.3, 47.14, 47.8, 49.8, 52.16, 67, 78.9, 119.1,
124.9, 127, 127.6, 141.3, 143.8, 155.2, 155.6, 174; HRMS calcd. for
C24H28N2Oþ6 ([M þ Na]þ): 463.4832; found: 463.4468.
General Procedure for Synthesis of 7 and 8
To a suspension of silica gel (10 g) in THF–water (20 mL, 1:1) was added
3b,c or 4b,c (5 mmol), and the resulting suspension was refluxed overnight.
After completion of the reaction, the suspension was filtered and the solvent
was removed under pressure. The resulting residue was diluted with EtOAc.
The combined organic layer was washed with brine and dried over
anhydrous Na2SO4.
Data
S-2-Fluorenyl-9-methoxycarbonyl-3-[N-benzyloxycarbonyl]propionic acid
(7a). 1.9 g, 83%, white solid, mp: 1198C; [a]2D5 þ 12.8; IR (KBr): 3371,
1
1715; H NMR (300 MHz, CDCl3): 3.45–3.64 (2H, br, CH2), 4.23 (1H, t,
CH), 4.36 (1H, dd, CH), 4.41 (2H, d, CH2), 5.23–5.32 (2H, br, CH2), 5.51
(1H, br, NH), 7.3–7.75 (13H, m, aromatic); 13C NMR (75 MHz, CDCl3): 47.8,
48.7, 56.2, 65.2, 78.1, 128.2, 128.5, 135.3, 141.1, 143.2, 155.8, 156.2, 174.5;
HRMS calcd. for C26H24N2Oþ6 ([M þ Na]þ): 483.4736; found: 483.4734.

2910
R. V. R. Rao, S. J. Tantry, and V. V. S. Babu
S-2-Fluorenyl-9-methoxycarbonyl-3-[N-allyloxycarbonyl] propionic acid
(8a). 1.76 g, 86%, white solid, mp: 1658C; [a]2D5 þ 9.2; IR (KBr): 3371,
1
1715; H NMR (300 MHz, CDCl3): 3.4–3.64 (2 H, br, CH2), 4.23 (1 H, t,
CH), 4.33 (1 H, dd, CH), 4.41 (2 H, d, CH2), 4.6 (2 H, d, CH2), 5.26–5.3
(1 H, m, CH), 5.48 (1 H, br, NH), 5.86–5.9 (2 H, m, CH2), 7.2–7.8 (8 H, m,
aromatic), 13C NMR (75 MHz, CDCl3): 42.4, 46.9, 55.0, 66.1, 66.5, 67.3,
117.9, 119.9, 125.1, 127.0, 127.6, 132.3, 141.1, 143.5, 156.6, 157.4; HRMS
calcd. for C22H22N2Oþ6 ([M þ Na]þ): 433.4138; found: 433.3842.
S-2-Fluorenyl-9-methoxycarbonyl-4-[N-benzyloxycarbonyl]butanoic acid
(8b). 1.91 g, 81%, white solid, mp: 1188C; [a]2D5 2 26.1; IR (KBr): 3371,
1715; 1H NMR (300 MHz, CDCl3): 1.9–2.1 (2 H, brd, CH2), 2.95–3.52
(2 H, brd, CH2), 4.23 (1 H, t, CH), 4.35 (1 H, dd, CH), 4.41 (2 H, d, CH2),
5.18 (2 H, br, CH2), 5.25 (1 H, br, NH), 5.4 (1 H, br, NH), 7.2–7.8 (13 H, m,
aromatic); 13C NMR (75 MHz, CDCl3): 47.14, 47.8, 49.8, 52.14, 66.8, 78.8,
118, 124.9, 127, 127.6, 141.3, 143.8, 155, 155.6, 174; HRMS calcd. for
C27H26N2Oþ6 ([M þ Na]þ): 497.5004; found: 497.1675.
S-2-Fluorenyl-9-methoxycarbonyl-4-[N-allyloxycarbonyl]butanoic acid (8b).
1.78 g, 84%, white solid, mp: 1248C; [a]2D5 2 22.8; IR (KBr): 3371, 1715; 1H
NMR (300 MHz, CDCl3): 1.9–2.1 (2H, br, CH2), 2.95–3.52 (2H, brd, CH2),
4.2 (1H, t, CH), 4.35 (1H, dd, CH), 4.41 (2H, d, CH2), 4.6 (2H, d, CH2), 5.27
(1H, q, CH), 5.4 (1H, br, NH), 5.86–5.9 (2H, m, CH2), 7.2–7.8 (8H, m,
aromatic); 13C NMR (75 MHz, CDCl3): 22.1, 40.4, 47.0, 55.1, 66.1, 66.5,
67.3, 117.9, 119.6, 125.5, 127.2, 127.6, 132.4, 141.2, 143.6, 156.6, 157.4,
170.4; HRMS calcd. for C23H24N2O6þ ([M þ Naþ): 447.4406; found: 447.4474.
General Procedure for the Synthesis of 10
To a 1 mmol of 4a or 4b in 10 ml of MeOH was added l eq. of 1 N of LiOH at
room temperature, and the mixture was stirred for 4 min and then neutralized
with 10% citric acid solution. The solvent was removed under reduced
pressure and the compound extracted twice into ethyl acetate (10 ml). The
combined organic layer was washed with a brine solution and dried over
anhydrous Na2SO4. It was concentrated and the resulting residue recrystal-
lized using ethylacetate–hexane (1:9).
Data
Methyl (2S)-3-[2-(benzyloxy)acetyl]amino-2[(9H-9-fluorenylmethoxy)-
carbonyl] aminopropanoate (10a). 0.346 g, 73%, white solid; mp: 1108C;
1
[a]2D5 þ 4.0; IR (KBr): 3375, 1750, 1701; H NMR (300 MHz, CDCl3): 3.57
(2H, S, CH2), 3.7 (3H, s, CH3), 4.21 (1H, t, CH), 4.24 (1H, d, CH), 4.38

Synthesis of 2,3-Dap and 2,4-Dab and their Derivatives
2911
(2H, d, CH2), 5.09 (2H, d, CH2), 5.38 (1H, brs, NH), 6.1 (1H, brs, NH), 7.27–
7.9 (13H, m, aromatic); 13C NMR(75 MHz, CDC3): 47.8, 48.9, 55.6, 65.6,
78.1, 128.2, 128.5, 135.3, 141.3, 143.2, 115.8, 156.2, 174.5. HRMS calcd.
for C27H26N2Oþ6 ([M þ Na]þ): 497.5004; found: 497.4968.
Methyl (2S)-4-[2-(benzyloxy)acetyl]amino-2-[(9H-9-fluorenylmethoxy)car-
bonyl] aminobutanoate (10b). 0.356 g, 73%, white solid, mp: 1128C;
1
[a]2D5 2 14.5; IR (KBr): 3370, 1745, 1709; H NMR (300 MHz, CDCl3): 1.7
(1H, m, CH2), 2.0 (1H, m, CH3), 2.9 (1H, m, CH2), 3.4 (1H, m, CH2). 3.68
(3H, s, CH3), 4.23 (1H, t, CH3), 4.34 (1H, m, CH), 4.41 (2H, d, CH2), 5.05
(2H, dd, CH3), 5.2 (1H, brs, NH), 5.4 (1H, d, NH), 7.27–7.9 (13H, m,
aromatic), 13C NMR(75 MHz, CDC3): 47.8, 48.7, 56.2, 65.8, 78.4, 128.2,
128.6, 135.2, 135.5, 141.3, 143.2, 156, 156.4, 174; HRMS calcd. for
C28H28N2Oþ6 ([M þ Na]þ): 511.5272; found: 511.4736.
General Procedure for the Synthesis of 11
The compound 4a,b (1 mmol) was dissolved in 5 ml of dry CHCl3, and
5 ml of trifluroacetic acid and triethylsilane (3 mmol) were added. The
solution was stirred at room temperature for 2 days. Concentration in
vacuo and reconcentration twice with CHCl3 gave a pale green oil. Recrys-
tallization of the residue using ethyl acetate–hexane (1:9) yielded a crystal-
line solid.
Data
(2S)-3-[2-Benzyloxy)acetyl]amino-2-[[(9H-9-flurenylmethoxy)carbonyl]
(methyl)aminolpropanoic acid (11a). 0.33 g, 70%, white solid: mp: 688C;
[a]2D5 þ 15; IR (KBr): 1715, 3370; 1H NMR (300 MHz, CDCl3): 2.93 (3H, s,
CH3), 3.64 (2H, brd, CH2), 4.21 (IH, t, CH), 4.24 (IH, d, CH), 4.38 (2H, d,
CH2), 5.08 (2H, d, CH2), 4.66 (IH, d, NH), 7.3–7.76 (13H, m, aromatic); 13
C
NMR (75 MHz, CDCl3): 47.8, 48.9, 56.2, 65.2, 78.1, 80.6, 128.2, 128.5,
135.3, 141.1, 143.2, 145.8, 156.2, 174.5; HRMS calcd. for C26H27N2O6þ
([M þ Na]þ): 497.5004; found: 497.4668.
(2S)-4-[2-benzyloxy)acetyl]amino-2-[[(9H-9-fluroenylmethoxy)carbonyl]
(methyl) amino]butanoic acid (11b). 0.341 g, 7%, white solid; mp: 62%;
1
[a]2D5 2 25; IR (KBr): 3371, 1713; H NMR (300 MHz, CDCl3): 1.9–2.1
(2H, brd, CH2), 2.91 (3H, s, CH3), 2.95–3.52 (2H, brd, CH2, 4.23 (1H, t,
CH), 4.34 (1H, m, CH), 4.41 (2H, d, CH2), 5.12 (2H, s, CH2), 5.25 (1H, br,
NH), 7.2–7.8 (13H, m, aromatic); 13C NMR (75 MHz, CDCl3): 47.5, 48.7,
56.2, 65.8, 78.4, 80.4, 128, 128.6, 135.5, 141.3, 143.2, 156, 156.4, 174;
HRMS calcd. for C28H28N2Oþ6 ([M þ Na]þ): 511.5272; found: 511.5238.

2912
R. V. R. Rao, S. J. Tantry, and V. V. S. Babu
ACKNOWLEDGMENTS
The authors are grateful to the Department of Bio-Technology, government of
India, for financial support. One of the authors (R. V. R. R.) is grateful to the
Council of Scientific and Industrial Research (CSIR) for the award of a
fellowship.
REFERENCES
1. Hettinger, T. P.; Craig, L. C. Biochemistry 1970, 9, 521.
2. Yoshioka, H.; Aoki, T.; Goko, H.; Nakatsu, K.; Noda, T.; Sakakibara, H.; Take, T.;
Nagata, A.; Abe, J.; Wakamiya, T.; Shiba, T.; Kaneko, T. Tetrahedron Lett. 1971,
12, 2043.
3. Takita, T.; Muraoka, Y.; Nakatani, T.; Fujii, A.; Umezawa, Y.; Naganawa, H.
J. Antibiot. 1978, 31, 801.
4. Scholtz, J. M.; Bartlett, P. A. Synthesis 1989, 542.
5. Wegner, M.; Rapoport, H. J. Org. Chem. 1977, 42, 3065.
6. Kretsinger, J. K.; Schneider, J. P. J. Am. Chem. Soc. 2003, 125, 1707.
7. Deng, J.; Hamada, Y.; Shioiri, T. Tetrahedron Lett. 1996, 37, 2261.
8. Englund, E. A.; Gopi, H. N.; Appella, D. H. Org. Lett. 2004, 6, 213.
9. Patil, B. S.; Vasanthakumar, G. R.; Suresh Babu, V. V. J. Org. Chem. 2003,
68; 7274:
10. Patil, B. S.; Suresh Babu, V. V. Lett. Pep. Sci. 2003, 10, 93.
11. Duggan, M. E.; Imagiri, J. S. Synthesis 1989, 131.
12. Vartak, A. P.; Young, V. G. Jr.; Johnson, R. L. Org. Lett. 2005, 7, 35.
13. Zhenyu, Z.; Aerschot, A. V.; Hendrix, C.; Busson, R.; David, F.; Sandra, P.;
Herdewijn, P. Tetrahedron 2000, 56, 2513.